This invention relates to antifuses, and more particularly, to electrically-programmable integrated circuit antifuses and methods for programming antifuses and sensing their states.
Programmable fuses and antifuses are used in a variety of integrated circuit applications. For example, a programmable logic device may have logic that is customized by programming appropriate fuses or antifuses on the device. Fuses and antifuses may also be used to permanently switch redundant circuitry into place to fix reparable defects during the integrated circuit manufacturing process. Sometimes it may be desired to use fuses or antifuses to program a serial number or other special information into a circuit.
Fuses and antifuses may be programmed using special laser-based systems or may programmed electrically.
With laser programming, each fuse or antifuse must be programmed in series, so the programming process can be lengthy. The programming equipment used in laser-based systems may also be complex and expensive.
Electrically programmable fuses and antifuses are attractive because the need for laser-based programming equipment is eliminated and programming speeds can generally be increased. Such a fuse or antifuse should have a high ratio of programmed to unprogrammed resistance and should be capable of being programmed with low power in a short amount of time. For applications in which it is desired to keep the state of the fuses or antifuses secret after programming, such as when fuses or antifuses are used to store cryptographic information, it should be difficult or impossible to use diagnostic tools such as scanning electron microscopes to determine whether a given fuse or antifuse has been programmed.
It is an object of the present invention to provide improved programmable integrated circuit antifuses whose state is difficult to ascertain by reverse engineering.